Energy absorbing, water blown, rigid polyurethane foam

ABSTRACT

The invention relates to energy absorbing, water blown, rigid polyurethane foams. The foams of the present invention are low density, predominantly open celled foams which exhibit a relatively constant consistency of compressive strength over deflection. Such foams are suitable as light weight alternatives in traditional energy absorbing applications.

This is a division of application Ser. No. 07/795,453, filed Nov. 21,1991.

FIELD OF THE INVENTION

The invention relates to energy absorbing rigid polyurethane foamcompositions. Specifically these are low density, predominantly opencelled, water blown rigid polyurethane foams which exhibit minimalspring back or hysteresis characteristics. Such foams are suitable aslightweight alternatives for traditional energy absorbing applications,such as side impact bolsters in automobile doors. The foams of thepresent invention exhibit properties comparable to energy absorbingfoams using chlorinated fluorocarbons as the blowing agent.

DESCRIPTION OF THE RELATED ART

U.S. Pat. No. 4,866,102 describes moldable energy absorbing rigidpolyurethane foam compositions which are prepared by the reaction of agraft polymer dispersion in a polyoxyalkylene polyether polyol with analkylene oxide adduct of toluenediamine or diaminodiphenylmethane withan organic polyisocyanate in the presence of a crosslinking agent.Similarily, U.S. Pat. No. 4,116,893; 4,190,712; 4,212,954; and 4,282,330also describe energy absorbing foams utilizing graft polymerdispersions. U.S. Pat. No. 4,722,946 describes the production of energyattenuating viscoelastic polyurethane elastomers and foams, comprisingmixtures of linear and branched polyol intermediates, polyisocyanates,and optionally, extenders, blowing agents, and the like, in the presenceof a catalyst whereby the isocyanate index is varied from about 65 toabout 85. U.S. Pat. No. 4,644,563 describes a method of shoring ageological formation which comprises preparing a rigid polyurethane foamwhich exhibits nearly constant strain with increasing stress incompression. U.S. Pat. No. 4,696,954 describes the preparation of moldedpolyurethane foams characterized by high impact strength and goodthermal stability. U.S. Pat. No. 4,615,754 describes a high densityrigid polyurethane foam which exhibits nearly constant strain withincreasing stress in compression. U.S Pat. No. 4,585,807 describes rigidpolyurethane foams employing oxyalkylated ethylenediamine.

SUMMARY OF THE INVENTION

The present invention relates to energy absorbing rigid polyurethanefoam compositions, the process for preparing said foams and, theresultant articles of manufacture, e.g. side impact bolsters forautomotive applications. The foams of the present invention arepredominantly open celled, energy absorbing, rigid polyurethane foamsessentially- free of chlorinated fluorocarbons and volatile organiccarbon blowing agents Yet the foams of the present invention havecharacteristics, such as constant crush strength and minimal spring backor hysteresis, comparable to present-day energy absorbing rigidpolyurethane foams containing fluorocarbons and volatile organiccarbons.

DETAILED DESCRIPTION OF THE INVENTION

Under ever increasing governmental regulations addressing both personalsafety and environmental concerns auto manufacturers have been put in aposition where they must meet stringent impact requirements, maintainvehicle weight and reduce the use of materials having a detrimentaleffect on the environment. Energy absorbing rigid polyurethane foamshave provided a partial solution in some impact performance requirementsareas; e.g. energy absorbing fillers which can be used to stiffen doorsupport frames, thus aiding in preserving the structural integrity ofthe passenger compartment of an automobile or as bolsters which canreduce effects of secondary collision (occupant-to-interior). However,the foams exhibiting the desired impact characteristics utilizechlorinated fluorocarbons as the foaming agent. That fact alone reducestheir desirability in light of mandates to reduce and eventuallyeliminate the use of CFC's.

In the instant invention it has been found that certain water blownrigid polyurethane foams can be produced which exhibit energy absorbingcharacteristics comparable to the CFC blown rigid polyurethane foams.These foams may be employed in energy absorbing applications, such asside impact bolsters in automobiles.

These foams are predominantly open celled; having molded densitiesranging from 2.0 pcf to about 4.5 pcf, and a crush strength whichremains constant from about 10% to about 70% in loading of up to about70 psi.

Previously, the foams exhibiting the desired impact characteristicsutilized chlorinated fluorocarbons as the foaming agent. Attempts tosubstitute water into these formulations failed to produce foamsexhibiting the same impact characteristics. Some success has been hadwith certain narrowly defined formulations using water as the blowingagent, and containing a polymer polyol (graft polyol) as necessaryelements of the invention. (See co-pending application U.S. Ser. No.634,643 filed Dec. 27, 1990, also U.S Pat. Nos. 4,190,712; 4,212,954 and4,116,893).

Surprisingly, it has now been found that foams exhibiting the desiredenergy absorbing characteristics may be produced from active hydrogencontaining compounds or mixtures thereof, organic isocyanates, water,and additives with the proviso that such compounds are generally capableof forming a rigid polyurethane structure, the critical nature of theinvention being the amount of water used in conjunction with theisocyanate and resin portions of a given rigid polyurethane foamformulation. The water must be present in amounts sufficient to act as ablowing agent and a cell opening agent promoting a predominantly opencell structure, yet not be present in amounts great enough to cause thefoam to collapse. The amount of water used is highly dependent upon theother formulation components but it has been found that from about 2weight percent to about 30 weight percent is an effective range. Apreferred range of water being from about 6 weight percent to about 15weight percent.

The polyisocyanates which may be used in the present invention aremodified and unmodified polyisocyanates which are well known to thoseskilled in the art. For the purposes of this invention the termpolyisocyanate is used to describe compounds containing, at least twoisocyanate groups. Unmodified polyisocyanates include aliphatic orcycloaliphatic and aromatic polyisocyanates. Examples include 2,4- and2,6-methylcyclohexylenediisocyanate; tetramethylene diisocyanate,cyclohexane diisocyanate, 2,4-toluene diisocyanate, 2,6 toluenediisocyanate, mixtures of 2,4- and 2,6-toluene diisocyanate,naphthalene-1,5-diisocyanate, 1-methoxyphenyl-2,4-diisocyanate.Preferred isocyanates include 4,4'-diphenylmethane diisocyanate (MDI),mixtures of 4,4'- and 2,4-diphenylmethane diisocyanate, and polymericpolyisocyanates such as polymethylene polyphenylenes polyisocyanates(polymeric MDI). Included within the preferred isocyanates are thosemodified MDI's containing carbodiimide, allophanate, urethane orisocyanurate structures. The more preferred isocyanates are polymericMDI and mixtures of polymeric MDI and pure 2,4 and 4,4' MDI. Thesepolyisocyanates are prepared by conventional methods known in the art,e.g. phosgenation of the corresponding organic amine.

For purposes of the present invention isocyanates other than thepreferred isocyanates may be present in minor amounts.

In the preparation of the polyurethanes of the present invention theisocyanate is reacted with active hydrogen-containing compounds (polyolsare preferred) Hydroxyl group-containing compounds (polyols) useful inthe preparation of polyurethanes are described in the PolyurethaneHandbook in chapter 3, §3.1 pages 42-61; and in Polyurethanes: Chemistryand Technology in Chapter II, §§III and IV, pages 32-47. Manyhydroxyl-group containing compounds may be used, including simplealiphatic glycols, dihydroxy aromatics, bisphenols, andhydroxyl-terminated polyethers, polyesters, and polyacetals, amongothers. Extensive lists of suitable polyols may be found in the abovereferences and in many patents, for example in columns 2 and 3 of U.S.Pat. No. 3,652,639; columns 2-6 of U.S. Pat. No. 4,421,872; and columns4-6 of U.S. Pat. No. 4,310,632; these three patents being herebyincorporated by reference.

Preferably used are hydroxyl-terminated polyoxyalkylene polyols. Theformer are generally prepared by well known methods, for example by thebase catalyzed addition of an alkylene oxide, preferably ethylene oxide(oxirane), propylene oxide (methyloxirane) or butylene oxide(ethyloxirane) to an initiator molecule containing on the average two ormore active hydrogens. Examples of preferred initiator molecules aredihydric initiators such as ethylene glycol, propylene glycol, butyleneglycol, neopentyl glycol, 1,6-hexanediol, hydroquinone, resorcinol, thebisphenols, aniline and other aromatic monoamines, aliphatic monoamines,and monoesters of glycerine; trihydric initiators such as glycerine,trimethylolpropane, trimethylolethane, N-alkylphenylenediamines, mono,di, and trialkanolamines; tetrahydric initiators such as ethylenediamine, propylene diamine, 2,4, 2,2', and 4,4'-methylenedianiline,toluenediamine, and pentaerythritol; pentahydric initiators such asdiethylenetriamine; and hexahydric and octahydric initiators such assorbitol and sucrose.

Addition of alkylene oxide to the initiator molecules may take placesimultaneously or sequentially when more than one alkylene oxide isused, resulting in block, heteric, and block-heteric polyoxyalkylenepolyethers The number of hydroxyl groups will generally equal the numberof active hydrogens in the initiator molecule. Processes for preparingsuch polyethers are described both in the Polyurethane Handbook andPolyurethanes: Chemistry and Technology as well as in many patents, forexample U.S. Pat. Nos. 1,922,451; 2,674,619; 1,922,459; 3,190,927; and3,346,557.

Polyester polyols also represent polyurethane-forming reactants. Suchpolyesters are well known in the art and are prepared simply bypolymerizing polycarboxylic acids or their derivatives, for exampletheir acid chlorides or anhydrides, with a polyol. Numerouspolycarboxylic acids are suitable, for example malonic acid, citricacid, succinic acid, glutaric acid, adipic acid, pimelic acid, azelaicacid, sebacic acid, maleic acid, fumaric acid, terephthalic acid, andphthalic acid Numerous polyols are suitable, for example the variousaliphatic glycols, trimethylolpropane and trimethylolethane,a-methylglucoside, and sorbitol. Also suitable are low molecular weightpolyoxyalkylene glycols such as polyoxyethylene glycol, polyoxypropyleneglycol, and block and heteric polyoxyethylene-polyoxypropylene glycols.These lists of dicarboxylic acids and polyols are illustrative only, andnot limiting. An excess of polyol should be used to ensure hydroxyltermination. Although carboxyl groups are also reactive withisocyanates. Methods of preparation of such polyester polyols are givenin the Polyurethane Handbook and in Polyurethanes: Chemistry andTechnology.

These polyols may be used alone or in combination. However, the use ofgraft polymer dispersions either alone or in combination is notcontemplated by this invention. Those polyols or mixtures of polyolsselected should promote a high crosslink density for optimal results.

Any suitable catalyst or mixture of catalysts may be used includingtertiary amines such as, for example, triethylenediamine,N-methylmorpholine, N-ethylmorpholine, diethylethanolamine,N-cocomorpholine, 1-methyl-4-dimethylaminoethylpiperazine,3-methoxypropyldimethylamine, N,N,N'-trimethylisopropy propylenediamine,3-diethylaminopropyldiethylamine, dimethylbenzylamine, and the like.Other suitable catalysts are, for example, stannous chloride,dibutyltin-di-2-ethyl hexonate, potassium hexanoate, stannous oxide, aswell as other organometallic compounds such as are disclosed in U.S.Pat. No. 2,846,408.

A surface-active agent is generally necessary for production of highgrade polyurethane foam according to the present invention, since in theabsence of same, the foams collapse or contain very large uneven cells.Numerous surface-active agents have been found satisfactory. Nonionicsurface active agents are preferred. Of these, the nonionicsurface-active agents such as the well-known silicones have been foundparticularly desirable Other surface-active agents which are operative,although not preferred, include polyethylene glycol ethers of long chainalcohols, tertiary amine or alkanol amine salts of long chain alkyl acidsulfate esters, alkyl sulfonic esters, and alkyl arylsulfonic acids

A chain extender and/or crosslinker is used as well in the presentinvention. These include those compounds having at least two functionalgroups bearing active hydrogen atoms such as, hydrazine, primary andsecondary diamines, amino alcohols, amine acids, hydroxy acids, glycolsor mixtures thereof Glycerin is an example of a preferred compound usedas a crosslinker.

Other optional additives which fall within the spirit of the presentinvention include known pigments, such as carbon black, dyes,stabilizers against aging and weathering, fungistats, bacteriostats,fillers, or flame retarding agents.

If desired, flame retardants may be incorporated in the foams. Among theflame retardants which may be employed are: pentabromodiphenyl oxide,dibromopropanol, tris(β-chloropropyl)phosphate, 2,2-bis(bromoethyl)1,3-propanediol, tetrakis(2-chloroethyl-)ethylene diphosphate,tris(2,3-dibromopropyl)phosphate, tris(β-chloroethyl)phosphate,tris(1,2-dichloropropyl)phosphate, bis-(2-chloroethyl)2-chloroethylphosphonate, molybdenum trioxide, ammonium molybdate,ammonium phosphate, pentabromodiphenyl oxide, tricresylphosphate,hexabromocyclododecane and dibromoethyl dibromocyclohexane.

The following examples illustrate the nature of the invention. All partsare weight % unless otherwise indicated.

Examples 1-3 and comparative examples A and B were prepared using thefollowing conditions:

    ______________________________________                                        Processing Conditions                                                         ______________________________________                                        Machine             High Pressure                                             Component Temp                                                                Resin °F.     77                                                       Isocyanate °F.                                                                              77                                                       Mixing Pressure                                                               Resin (bar)         180                                                       Isocyanate (bar)    180                                                       Throughput g/sec    175                                                       Mold Temperature °F.                                                                       100 ± 5                                                Mold Release        Wax base release                                                              agent*                                                    Processing Mode     Open Mold Pour                                            Demold Time min     ˜3                                                  ______________________________________                                         *Silicone was used as the mold release for comparative examples A and B. 

Examples 1-3 and Comparative example A and B were injected into thecenter of a preheated 10×10×2.5" mold. The mold was closed. The partswere demolded, cured at 50% humidity and 73° F. prior to testing. Foamcompression testing (ASTM D-1621) at 10 percent intervals was carriedout on all examples at varying full weights, to determine foamperformance characteristics. Comparative example A is a currently usedenergy absorbing foam composition using Freon F-11a (a chlorinatedfluorocarbon). Comparative example B is a formulation similar tocomparative example A however water is substituted for F-11a.

Although these parts were made using an open mold pour technique, thesystem is equally amenable to other fabrication methods known to thoseskilled in the art, e.g. RIM (reaction injection molding).

Polyol A was a propylene oxide adduct of a sucrose/dipropylene glycolmixed initiator, having a molecular weight of about 620 and a hydroxylnumber of about 397.

Polyol B was a propylene oxide adduct of ethylene diamine having amolecular weight of about 293, and a hydroxyl number of 767.

Polyol C was a propylene oxide adduct of a mixture of toluene diamineisomers containing predominantly vicinal isomers, having a molecularweight of about 570 and a hydroxyl number of about 390.

Polyol D was a propylene oxide adduct of propylene glycol containing 50percent of 2.1 acrylonitrile/styrene, having a hydroxyl number of about69.

DC 193 was a silicone surfactant sold by Dow Corning.

TEGOSTAB B-4113 was a silicone surfactant sold by Goldschmidt.

NIAX L-540 was a silicone surfactant sold by Union Carbide.

NIAX C-174 was an amine catalyst sole by Union Carbide.

DABCO R-8020 was an amine catalyst sold by Air Products.

DAECO 33LV was an amine catalyst sold by Air Products.

POLYCAT 5 was an amine catalyst sold by Air Products.

HexChem 977 was an organometallic catalyst, potassium octoate, indipropylene glycol.

FOMREZ UL-1 was a tin catalyst sold by Fomrez Corp.

F11A was a chlorinated fluorocarbon sold by Dupont Corporation.

ISO A was a polymeric polymethylene polyphenyl isocyanate having afunctionality of about 2.7.

All reported test values are averages of multiple on each sample.

    ______________________________________                                                    Examples                                                          Resin Component                                                                             1       2       3     A    B                                    ______________________________________                                        Polyol A      82.5    83.39   --    --   --                                   Polyol B      --      --      67.0  --   --                                   Polyol C      --      --      --    21.56                                                                              29.29                                Polyol D      --      --      --    30.81                                                                              41.84                                DC-193        0.41    0.41    --    --   --                                   Tegostab B-4113                                                                             --      --      0.4   --   --                                   Niax L-540    --      --      --    0.49 1.25                                 Niax C-174    --      --      --    --   2.1                                  DABCO R-8020  --      --      --    0.92 --                                   DABCO 33LV    --      --      0.6   --   --                                   Polycat 5     0.5     0.5     --    --   --                                   Hexchem 977   0.83    0.83    2.0   --   --                                   Fomrez UL-1   --      --      --    0.02 --                                   Glycerin      8.26    8.26    15.0  9.24 12.55                                Water         7.44    6.61    15.0  --   12.55                                F-11A         --      --      --    36.96                                                                              --                                   Isocyanate Component                                                          Iso A         200     200     200   89.3 119.5                                ______________________________________                                    

                                      TABLE 1                                     __________________________________________________________________________                     STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                     Density                                                                              Strength                                                                           AT 10% AT 20% AT 30% AT 40%                                       (pcf)  at Yield                                                                           CRUSH  CRUSH  CRUSH  CRUSH                                   Example                                                                            Molded/Core                                                                          (psi)                                                                              (psi)  (psi)  (psi)  (psi)                                   __________________________________________________________________________    1    3.15/2.20                                                                            22.66                                                                              22.97  24.33  26.46  27.98                                   1     3.4/2.44                                                                            25.96                                                                              26.32  27.37  28.86  29.75                                   1    3.67/2.58                                                                            32.94                                                                              32.84  33.29  33.88  34.40                                   1    3.81/2.88                                                                            35.05                                                                              35.20  36.50  37.08  37.98                                   1    4.09/3.13                                                                            38.12                                                                              38.13  36.77  36.54  37.76                                   1    4.38/3.30                                                                            50.78                                                                              50.21  50.47  50.21  50.84                                   __________________________________________________________________________                     STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                 AT 50% AT 60% AT 70% AT 80%                                                   CRUSH  CRUSH  CRUSH  CRUSH                                               Example                                                                            (psi)  (psi)  (psi)  (psi)                                   __________________________________________________________________________                1    29.03  30.07  30.66  64.53                                               1    30.73  32.05  31.57  69.84                                               1    35.17  36.21  38.25  86.18                                               1    39.30  40.45  40.43  92.28                                               1    38.82  40.13  44.08  100.30                                              1    51.49  53.29  56.35  129.5                                   __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                     STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                     Density                                                                              Strength                                                                           AT 10% AT 20% AT 30% AT 40%                                       (pcf)  at Yield                                                                           CRUSH  CRUSH  CRUSH  CRUSH                                   Example                                                                            Molded/Core                                                                          (psi)                                                                              (psi)  (psi)  (psi)  (psi)                                   __________________________________________________________________________    2    2.78/2.37                                                                            27.39                                                                              25.92  27.14  27.20  27.98                                   2    3.12/2.21                                                                            36.92                                                                              36.41  38.41  38.55  39.25                                   2    3.33/2.88                                                                            41.72                                                                              40.61  41.75  41.60  41.68                                   2    3.49/3.04                                                                            46.10                                                                              46.45  48.12  48.09  48.75                                   2    3.73/3.26                                                                            53.74                                                                              53.17  55.56  54.19  54.40                                   2    3.99/3.51                                                                            62.04                                                                              62.73  62.25  61.32  61.72                                   __________________________________________________________________________                     STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                 AT 50% AT 60% AT 70% AT 80%                                                   CRUSH  CRUSH  CRUSH  CRUSH                                               Example                                                                            (psi)  (psi)  (psi)  (psi)                                   __________________________________________________________________________                2    28.86  29.25  39.53   72.34                                              2    39.97  41.33  46.45   94.64                                              2    42.71  44.40  50.78  104.5                                               2    50.36  52.04  54.86  116.3                                               2    55.61  57.40  61.76  134.5                                               2    62.84  65.26  68.05  152.2                                   __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                     STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                     Density                                                                              Strength                                                                           AT 10% AT 20% AT 30% AT 40%                                       (pcf)  at Yield                                                                           CRUSH  CRUSH  CRUSH  CRUSH                                   Example                                                                            Molded/Core                                                                          (psi)                                                                              (psi)  (psi)  (psi)  (psi)                                   __________________________________________________________________________    3    3.39/2.03                                                                            8.4  9.1    9.6    10.4   11.6                                    3    3.94/2.37                                                                            16.7 14.8   14.6   14.6   15.6                                    __________________________________________________________________________                     STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                 AT 50% AT 60% AT 70% AT 80%                                                   CRUSH  CRUSH  CRUSH  CRUSH                                               Example                                                                            (psi)  (psi)  (psi)  (psi)                                   __________________________________________________________________________                3    13.5   14.6   15.7   17.1                                                3    16.9   18.2   19.2   27.0                                    __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Comparative Examples A & B                                                    __________________________________________________________________________         Core      STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                       Density                                                                            Strength                                                                           AT 10% AT 20% AT 30% AT 40%                                         2.5-2.8                                                                            at Yield                                                                           CRUSH  CRUSH  CRUSH  CRUSH                                     Example                                                                            (pcf)                                                                              (psi)                                                                              (psi)  (psi)  (psi)  (psi)                                     __________________________________________________________________________    A         17.84                                                                              16.57  15.10  15.70  16.34                                     B         --   37.46  39.80  42.40  45.77                                     __________________________________________________________________________                  STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                 AT 50% AT 60% AT 70% AT 80%                                                   CRUSH  CRUSH  CRUSH  CRUSH                                               Example                                                                            (psi)  (psi)  (psi)  (psi)                                      __________________________________________________________________________             A    17.37  19.18  26.92  85.59                                               B    51.40  61.77  80+    100+                                       __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                    STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                            Strength                                                                           AT 10% AT 20% AT 30% AT 40%                                        Fill Weight                                                                         at Yield                                                                           CRUSH  CRUSH  CRUSH  CRUSH                                    Example                                                                            (grams)                                                                             (psi)                                                                              (psi)  (psi)  (psi)  (psi)                                    __________________________________________________________________________    1    263   20.8 19.9   21.4   23.3   25.1                                     1    276   26.3 25.9   27.8   29.5   31.3                                     1    300   30.5 30.0   32.1   34.0   36.1                                     1    314   38.8 39.0   37.0   37.0   38.1                                     1    352   57.4 51.2   50.0   50.7   52.1                                     __________________________________________________________________________                    STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                                             STRENGTH                                                 AT 50% AT 60% AT 70% AT 80%                                                   CRUSH  CRUSH  CRUSH  CRUSH                                               Example                                                                            (psi)  (psi)  (psi)  (psi)                                    __________________________________________________________________________               1    26.9   28.2   33.6   67.4                                                1    32.9   34.3    43.45 86.5                                                1    37.8   39.6   47.2   96.0                                                1    39.3   42.8   53.3   105.64                                              1    55.4   59.9   69.7   137.2                                    __________________________________________________________________________

Examples 1, 2 and 3 illustrate the present invention. Examples 1 and 2are prepared using the same formulation with different amounts of waterpresent. The resulting foams exhibit the desired characteristics of theinvention, i.e. a relatively constant consistency of compressivestrength or crush strength, over deflection (ASTM D-1621). Thisconsistency of compressive strength is carried over a range of densitiesas shown in tables 1 and 2. Example 3 illustrates that energy absorbingrigid foams are possible by adjusting the amount of water necessary toproduce foams having the desired physical characteristics.

Table 3 shows the results of ASTMD-1621 foam compression tests for foamsproduced from example 3.

Table 4 shows the results of ASTM D-1621 foam compression test forcomparative examples A and B. As is evident from the data merelysubstituting water (Ex. B) for a chlorinated fluorocarbon (Ex. B) in aformulation produces a foam which exhibits a significant loss inefficiency, i.e. consistency of compressive strength over deflection.

Table 5 shows test data from a prototype hip bolster part, covering therange of 20 to 50 psi by varying the fill weight. The hip bolster wasprepared using the formulation from example 1. All conditions were thesame as those for examples 1 through 3 with the exception that a hipbolster mold was used.

We claim:
 1. An energy absorbing polyurethane foam, comprising thereaction product of:A) a compound containing isocyanate reactivehydroxyl groups selected from the group consisting of aliphatic glycols,dihydroxy aromatics, bisphenols, hydroxyl terminated polyethers,polyesters, polyacetals and mixtures thereof, B) an organic isocyanateselected from the group consisting of 4,4'-diphenylmethane diisocyanate,polymethylene polyphenylene polyisocyanate, modified diphenylmethanediisocyanates, and mixtures thereof, wherein other organic isocyanatesmay be present in minor amounts, C) a catalyst, D) a blowing agentconsisting of water, wherein the water is present in amounts sufficientto cause formation of a predominantly open cell foam, without causingcollapse, E) a surfactant, and F) a crosslinker,wherein said reactionproduct has a molded density of from about 2.0 pcf to about 4.5 pcf anda crush strength which remains constant from about 10 percent to about70 percent deflection at loads less than 70 psi.
 2. A composition asclaimed in claim 1, wherein the compound containing isocyanate reactivehydroxyl groups is a hydroxyl-terminated polyoxyalkylene polyol.
 3. Acomposition as claimed in claim 1, wherein the compound containing theisocyanate reactive group is a propylene oxide adduct of asucrose/dipropylene glycol mixed initiator having a molecular weight ofabout 620 and a hydroxyl number of about
 400. 4. A composition asclaimed in claim 1, wherein the isocyanate is a polymeric polymethylenepolyphenylisocyanate.
 5. A composition as claimed in claim 1, whereinthe surfactant is a silicone surfactant.
 6. A composition as claimed inclaim 1, wherein the crosslinker is a glycerin.
 7. A composition asclaimed in claim 1, wherein the water is present in amounts from about 2wt.% to about 30 wt.%.
 8. A composition as claimed in claim 9 whereinthe water is present in amounts from about 6 wt.% to about 15 wt.%. 9.An energy absorbing rigid polyurethane foam article having a moldeddensity of from about 2.0 pcf to about 4.5 pcf and a consistency ofcompressive strength from 10% to 70% deflection at loading of up toabout 70 psi, comprising the reaction product of;A) a polymethylenepolyphenylisocyanate having a functionality of about 2.7, B) A C₂ -C₄alkylene oxide adduct of an initiator selected from a group consistingof sucrose, dipropylene glycol and mixtures thereof, C) a catalyst, D)from about 6 wt.% to about 15 wt.% water, E) a silicone surfactant, andF) glycerin.